1. FIELD OF THE INVENTION
The present invention concerns scintillation counters. More particularly the present invention is directed to alpha-beta-gamma scintillation spectrometers for the measurement of radioactive samples.
2. DESCRIPTION OF THE PRIOR ART
The core of an alpha-beta-gamma scintillation spectrometer comprises a cylindrical NaI(Tl) crystal with two photomultiplier tubes coupled to it and at the center of this crystal, perpendicular to its major axis, there is a cylindrical cavity covered with quartz. When a sample containing a beat emitting isotope is to be measured, it is dissolved in a liquid scintillator and the vial containing the radioactive liquid scintillator is loaded into this cavity. The light emitted from a liquid scintillator when a beta particle interacts with it is piped through the NaI(Tl) crystal to the two photomultiplier tubes. Only when the pulses from both of the p.m. tubes are in coincidence, and only when these are fast pulses, as determined by pulse shape analyzers, the signal is stored in a multichannel analyzer. The NaI(Tl) crystal serves not only as a light pipe but also as an anti-coincidence shield for reducing background counting rate. When a gamma, or a cosmic ray, interacts both with the liquid scintillator and the NaI(Tl) crystal, the "false" pulse is not counted. Such background pulses are rejected by pulse-shape discriminators. The light emitted from the liquid scintillator and the NaI(Tl) crystal reaches the same p.m. tubes. However, the pulse shape of a pulse which originates in the organic liquid scintillator differs from that which originates in the inorganic NaI(Tl) crystal; whereas the decay constant of a light pulse from the liquid scintillator is about 3 nsec, the decay constant of a light pulse from the NaI(Tl) crystal is about 230 nsec. By using pulse-shape discriminators it is possible to ascertain the origin of the pulse.
Alpha emitters are measured in the same way as beta emitters. When a gamma emitter is to be measured the NaI(Tl) crystal serves as a conventional scintillation detector.
The alpha-beta-gamma scintillation spectrometer is a universal spectrometer with which any kind of radioactive isotope can be measured. However, it was found out that this spectrometer is not suitable for the measurement of samples which contain low activities of tritium. Whereas the background in the tritium window with conventional liquid scintillation spectrometers is about 20 counts per minute, the background in this window with an alpha-beta-gamma scintillation spectrometer is over 200 counts per minute. It should be realized that this is quite a serious disadvantage as tritium is widely used, inter alia, in biological and medical laboratories, being the only radioactive tracer of hydrogen, the origin of this high background counting rate was not known up to now, nor was it known how this problem could be solved so as to make alpha-beta-gamma spectrometers useful in measuring low level tritium samples.